Speed up empty String creation slightly (#6573)
Speed up empty String creation slightly (#6573)

As @dirkmuller found out in #6568, there is a difference in code
executed between `String str(nullptr)` and `String str("")`, but in the
end the actual object is identical.  It's a few bytes of code, but every
little bit counts.

Update the default `String()` constructor to use `nullptr` and not `""`.
This will remove a constant literal load and the execution of the
String::copy method and strlen().
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